The presence of hydrogen in the cavity volume inside an electronic package, such as an integrated circuit package, is known to cause significant degradation of some type of semiconductor devices. For example, high frequency gallium arsenide (GaAs) Schottky field effect transistors (FETs) incorporating Schottky metal gates upon channels are susceptible to hydrogen impurities. While the channel of the gate could otherwise be marginally protected by a good insulating dielectric layer, the GaAs processes do not enable good insulating layer formation for making suitable FETs. This is unlike silicon process based FETs which can have good silicon oxide insulators grown as a gate to channel dielectric insulating layer. A good oxide gate dielectric insulator is desired so that when a gate voltage is applied, gate electrons do not flow into the channel from the gate. Insulating layers can also partially protect the channel from contamination. The chemical binary nature of Group III-V materials, e.g. GaAs, do not form good oxide layers for insulating purposes and are thus not well suited for dielectric layer formation within a FET structure. Germanium is a good semiconductor material with a higher electron mobility than the GaAs materials, but germanium oxide is hygroscopic and not well suited as insulating layers. GaAs may be used to form a gallium oxide layer but the presence of arsenic degrades the effectiveness of the gallium oxide insulating layer. Prior unsuccessful efforts have been directed to finding a suitable GaAs based insulator to prevent contamination of the channel while providing a suitable gate dielectric. Gallium oxide is not used as a FET gate dielectric. The GaAs FETs have used Schottky metal gates deposited directly on the GaAs channel. The Schottky junction is asymmetric between the GaAs channel and the metal gate and such FET devices have been in use for some time. However, the Schottky gate can communicate diffused hydrogen and impurities directly into the channel to thereby contaminate the channel.
Prior efforts have been directed towards eliminating unwanted hydrogen from electronic packages containing Schottky Gate GaAs FETs as well as many other electronic devices. The Schottky barrier forms a junction at the semiconductor and the metal layer interface. It is desirable to create stable surface structures in semiconductor devices including the GaAs Schottky gate-channel interface. The surface of the GaAs channel is not a consistent lattice structure and is characterized by gallium and arsenic sites. Certain metals are known to bond well and permanently to such irregular GaAs surface sites. For example, titanium and aluminum are known to bond well to the surface of the irregular GaAs FET channel. It is also desirable that the gate be connected to a highly conductive metal, such as gold or silver. Thus, a GaAs FET metal Schottky gate preferably consists of one or more metal layers, including a contact metal such as titanium or aluminum, and barrier metal, such as palladium or platinum, and a conduction metal, such as gold or silver. Several Schottky metal gate contact schemes have been used. When using a GaAs semiconductor, for example, a bottom bonding titanium metal layer is used as the Schottky contact metal, which is covered by a middle platinum barrier layer, which is covered by a top gold conduction layer. The platinum is used to prevent gold from diffusing into and through the titanium layer, as is well known. The problem with the use of such metal systems is the interaction of transition metals with hydrogen trapped in the electronic package. These transition metals, such as palladium and platinum, are known to interact with molecular hydrogen. These transition metals convert, at their surface, molecular hydrogen into atomic hydrogen which may then diffuse through the metal. Thus, a palladium or platinum metal barrier layer may collect hydrogen which then may disadvantageously diffuse through the barrier layer and through the contact metal, such as titanium, to the FET gate-channel boundary and as well as disadvantageously diffusing into the GaAs channel itself. A gold or silver conducting layer covering the barrier layer may seem to provide some protection from the collection and diffusion of hydrogen into the barrier layer, but the conducting layer does not completely cover the barrier layer particularly at the edges providing access of hydrogen to the barrier and contact layers as well as the junction and channel.
Hydrogen will diffuse into the metal gate-channel junction boundary or into the GaAs channel. It is believed that diffused atomic hydrogen causes unwanted electronic dipoles at the gate-channel Schottky junction to affect energy levels, or causes neutralization of dopant centers within the channel. For example, the affects are believed to cause as much as a forty percent reduction in drain to source current in a five percent hydrogen gas. The diffusion of atomic hydrogen, and to a lesser extent the diffusion of molecular hydrogen, degrades the gain and increases noise of GaAs FETs. Additionally, the hydrogen collection and diffusion process occurs over time to degrade the electronic performance of the GaAs FET over time, well after the initial fabrication of the FET. The presence of molecular hydrogen within the package continuously degrades the performance of the GaAs FETs. Thus, much effort has been expended to reduce gaseous molecular hydrogen within electronic semiconductor packages to prevent initial, continuous and long term degradation of the GaAs FET as well as other electronic devices.
Hermetic sealing of an electronic package is used to prevent contamination from unwanted materials such as organic materials, water, hydrocarbons, and ionic species. The sealing of an electronic package containing integrated circuits in an ultra pure inert gases does not eliminate the formation of hydrogen therein because of the materials often used in the package, including the electronic devices, evolve molecular hydrogen over time after package sealing. For example, nickel or silver electronic package plating materials, or for another example, microwave absorbing materials often contain traces of hydrogen which evolve over time into gaseous molecular hydrogen trapped within the electronic package and available for diffusion into integrated circuit semiconductor devices to potentially degrade integrated circuit performance over time.
Steps have been taken to eliminate the amount of trace hydrogen from the semiconductor materials prior to and during processing. However, all metals contain trace amounts of hydrogen. So even in the cleanest environments, hydrogen is still present in small impurity concentrations. Vacuum deposition processes of metals using very low vapor pressures, electrolysis deposition, pre-bake processes and ultra clean air environments will not eliminate the presence of hydrogen. Trace hydrogen will always be present in the manufacture of electronic devices. Therefore, there exists a need to create a means which will over time vent out unwanted molecular hydrogen from the internal cavity of electronic packages containing electronic and semiconductor devices, yet will also allow hermetic sealing of the packages.
Some laboratory apparatus have used catalytic metals such as palladium, to purify hydrogen gas using surface catalytic dissociation and recombination of molecular hydrogen. Such an apparatus has not been adapted to collect hydrogen within electronic packages. Some approaches have used hydrogen gettering materials within an electronic package to absorb internally trapped molecular hydrogen, but this approach does not eliminate the hydrogen but acts to capture the unwanted hydrogen. The absorbing gettering materials have absorption limits, may not completely absorb all of the trapped hydrogen and will evolve some trace hydrogen over time. These approaches have proven to be costly and ineffective in eliminating the presence of hydrogen within an electronic package. It is practically impossible to eliminate all trace elements of hydrogen.
Another approach applied in space exploration vehicles has been the use of out venting holes drilled or formed into the electronic packages so that the hydrogen within an electronic package will vent out over a period of time into the lower pressure of outer space once the electronic packages are placed in orbit. The problem with this approach is that the electronic devices are subject to contamination during terrestrial fabrication and prelaunch activities even with some marginal benefits of out venting of hydrogen to ambient air. However, the use of a hole in the package disadvantageously prevent hermetic sealing and enables the contamination of the internal circuits by unwanted materials.
Thus, there continues to exist a need to remove hydrogen from electronic packages even with advanced sealing and material purification techniques. The problem of sealing an electronic package to prevent contamination while enabling out venting of hydrogen has not been solved. The continued presence of hydrogen in electronic packages continues to disadvantageously degrade integrated circuit performance over time. These and other disadvantages of unwanted trapped hydrogen are solved or reduced using the present invention.